The Notch Signaling Pathway
The Notch signaling pathway has been identified as playing an important role in many diverse biological functions, including differentiation, and cellular proliferation (see U.S. Pat. No. 6,703,221). This pathway is activated by four different transmembrane receptor subtypes (designated as Notch1-Notch4) that rely on regulated proteolysis. Expression patterns and functions of Notch depend on cell type and context. Following ligand binding, the receptor undergoes sequential cleavage by metalloproteases of the ADAM family (Bru, et al., Mol. Cell 5:207-216 (2000); Mumm, et al., Mol. Cell 5:197-206 (2000)) and the presenilin-dependent gamma-secretase (Selkoe, et al., Annu. Rev. Neurosci. 26:565-97 (2003); De Strooper, et al., Nature 398:518-522 (1999)). The final proteolytic cleavage step permits the intracellular domain of the Notch receptor to translocate to the cell nucleus where it interacts with transcription factors to induce target gene expression.
In the cell nucleus, the Notch intracellular domain undergoes ubiquitilation. Proteolytic processing of the Notch precursor protein by furin-protease and its trafficking to the cell membrane also determine turnover and availability of receptors, and, in turn, activation of this signaling pathway. Altered glycosylation of the Notch extracellular domain by Fringe protein family members may also modify efficiency of ligand binding.
Notch Signaling and Macrophage Activation
The Notch pathway contributes to biological processes during development and to disease mechanisms in adults (Bray, et al., Nat. Rev. Mol. Cell. Biol. 7:678-689 (2006); Artavanis-Tsakonas, et al., Science 284:770-776 (1999)). Direct cell-to-cell contract via the binding of a ligand to a Notch receptor, both of which are expressed on the cell surface, triggers downstream responses (Thurston, et al., Nat. Rev. Cancer 7:327-331 (2007)). We previously demonstrated that Dll4-mediated Notch signaling promotes macrophage activation (Fung, et al., Circulation 115:2948-2956 (2007); Fukuda, et al., Proc. Nat'l Acad. Sci. USA 109:E1868-1877 (2012)). Clinical and preclinical evidence has established the causal role of macrophages in arterial atherosclerosis (Aikawa, et al., Cardiovasc. Pathol. 13:125-138 (2004); Moore, et al., Nat. Rev. Immunol. 13:709-721 (2013)). Failing vein grafts also tend to contain macrophages (Fowkes, et al., Lancet 382:1329-1340 (2013)) but their role in the disease progression remains unclear.
Vein Graft Failure
Vein graft failure is a global health burden with no effective medical solutions (Owens, et al., Vasc. Med. 13:63-74 (2008)). Due to the pandemic of atherosclerotic peripheral artery disease (PAD) and the growing prevalence of underlying metabolic disorders (Fowkes, et al., Lancet 382:1329-1340 (2013)) the incidence of vein graft failure is rising. Although many mechanisms for arterial diseases have been established, the pathogenesis of vein graft failure remains incompletely understood. Autologous saphenous vein grafts (SVGs) are widely used for PAD because they remain patent longer than artificial conduits (Twine, et al., Cochrane Database Syst. Rev. CD001487 (2010)). Approximately 50% of lower extremity SVGs, however, become occluded or narrowed within a year (Conte, et al., J. Vasc. Surg. 43:742-751 (2006). When PAD grafts fail, the only available therapeutic options are devastating limb amputation or invasive and expensive angioplasty or surgical revascularization. Coronary artery SVGs also fail at high rates (Fitzgibbon, J. Am. Coll. Cardiol. 28:616-626 (1996)). Although current therapies such as statins can reduce the onset of complications of arterial diseases (e.g., myocardial infarction) (Libby, et al., Nat. Med. 8:1257-1262 (2002)), no effective medical solutions are available for vein graft failure.